Contents lists available at ScienceDirect Tribology International journal homepage: www.elsevier.com/locate/triboint Temperature/shear-induced interface reconstruction of hot rolled carbon steel lubricated by polyphosphate composite lubricants Shanhong Wan a , Huynh H. Nguyen a , Sang T. Pham a , Anh Kiet Tieu a,* , Hongtao Zhu a,** , Thi Dinh Ta a , Qiang Zhu b a Faculty of Engineering and Information Sciences, University of Wollongong, Northfields Avenue, Wollongong, NSW, 2522, Australia b Electron Microscope Unit, The University of New South Wales, Sydney, NSW, 2006, Australia ARTICLE INFO Keywords: Hot rolling Alkaline polyphosphate Oxide nanoparticles Lubrication ABSTRACT The present work aims to investigate temperature/shear-induced oxide reconstruction at the hot rolled steel interface, where sodium/potassium polyphosphates blended with different oxide nanoparticles were applied as the lubricant to control friction and manage oxidation. The lubrication of alkaline polyphosphate reduces the rolling force by up to 15% and prevents oxidation significantly. The undesired oxide scale can be transformed into iron oxide/alkaline polyphosphate composite imposed on hot rolled steel. Addition of inorganic oxide particles improves the consolidation and uniformity of reconstructed oxide layer. The polyphosphate formula- tions demonstrate the superiority of eliminating red scale over silicon alloyed steel. The mechanism of tem- perature/shear-induced interfacial transformation of the hot rolled steel associated with the interactions be- tween oxide scale and alkaline polyphosphate constituent was discussed. 1. Introduction Refining hot rolling process through a tribological approach can potentially improve the cost reduction and product quality. The need for a more effective and adaptive lubrication during steel rolling has always been an increasingly interesting topic. Basically, the interactions at the roll-strip-lubricant contacts governs the efficiency of lubricating media, the service life of work roll, and strip surface quality. However, the roll-gap involves complex factors e.g. high normal and shear stresses, friction, oxide scale and lubricant [1], which affects the eventual rolling performance. In particular, the undesirable interactions leads to the severe mechanical damage to the roll, with the roll main- tenance cost accounts for 10% of the total rolling process [2]. Con- siderable studies have been carried out to evaluate various lubricants at the hot roll-strip contacts, in which the friction and wear behaviour, surface quality and the service life of roll are of significant interest [3–5]. Williams highlighted the significance of lubrication in the steel manufacturing from various European, American and Japanese sources which show the power consumption and rolling loads can be reduced by 15–25% and roll life increases from 20% to 40% in the hot rolling process [1,6]. Alkaline-based polymeric derivatives have been applied intensively in the hot rolling industry [7,8]. As compared to over-based sulfonates, alkaline polyphosphate compounds particularly possess the desirable melting temperature range, delivering the semi-fluidic lubricating ability at the frictional interface where the working temperature is around 800 °C [7,9]. Favourable friction and wear reduction of poly- phosphate-based lubricant have been attained from our laboratory ap- praisals. It is noted that alkaline polyphosphate has a highly chemical reactivity with oxide, which assures a strong adhesion onto the sub- strate and permits itself to form a protective film before hot rolling. An investigation further highlighted that, if the sliding contact pressure is too high, e.g. highly stressed shearing, such alkaline polyphosphate film cannot provide the sufficient protection of the roll-strip contact from wear and oxidation [10]. The amount of particulate additives probably contribute to the improved performance of lubricant base like oil/water by interposing a thin adherent layer with easy shearing or mending effect [11]. In this study, an addition of inorganic oxide particles into alkaline phosphate base is expected to further improve the lubricity of hot rolling of mild carbon steel and to clarify their working mechanism at elevated temperatures. During hot rolling process, steel specimens are subjected to cyclic thermal-mechanical effects. The stressed-shearing action can initiate complex chemical reactions between lubricant and solid surfaces, as https://doi.org/10.1016/j.triboint.2019.105863 Received 26 January 2019; Received in revised form 1 July 2019; Accepted 8 July 2019 * Corresponding author. ** Corresponding author. E-mail addresses: ktieu@uow.edu.au (A.K. Tieu), hongtao@uow.edu.au (H. Zhu). Tribology International 140 (2019) 105863 Available online 15 July 2019 0301-679X/ © 2019 Published by Elsevier Ltd. T